Chapter 22 Fundamentalfundamental Propertiesproperties Ofof Antennasantennas

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Chapter 2

Fundamental Properties of Antennas

ECE 5318/6352
Antenna Engineering
Dr. Stuart Long

. IEEE Standards

. Definition of Terms for Antennas

. IEEE Standard 145-1983 . IEEE Transactions on Antennas and
Propagation Vol. AP-31, No. 6, Part II, Nov. 1983

. Radiation Pattern
(or Antenna Pattern)

“The spatial distribution of a quantity which characterizes the electromagnetic field

generated by an antenna.”

. Distribution can be a

. Mathematical function . Graphical representation . Collection of experimental data points

. Quantity plotted can be a

. Power flux density W [W/m²] . Radiation intensity U [W/sr] . Field strength E [V/m]

. Directivity D

. Graph can be

. Polar or rectangular

. Graph can be

. Amplitude field |E| or power |E|² patterns

(in linear scale)
(in dB)

. Graph can be

. 2-dimensional or 3-D most usually several 2-D “cuts” in principle planes

. Radiation pattern can be

. Isotropic

Equal radiation in all directions (not physically realizable, but valuable for comparison purposes)

. Directional

Radiates (or receives) more effectively in some directions than in others

. Omni-directional

nondirectional in azimuth, directional in elevation

.Principle patterns

. E-plane
. H-plane

Plane defined by H-field and direction of maximum radiation
Plane defined by E-field and direction of maximum radiation

(usually coincide with principle planes of the coordinate system)

Coordinate System

Fig. 2.1 Coordinate system for antenna analysis.

. Radiation pattern lobes

. Major lobe (main beam) in direction of maximum radiation (may be more than one)

. Minor lobe - any lobe but a major one . Side lobe - lobe adjacent to major one . Back lobe – minor lobe in direction exactly opposite to major one

. Side lobe level or ratio (SLR)

. (side lobe magnitude / major lobe magnitude) . - 20 dB typical . < -50 dB very difficult
Plot routine included on CD for rectangular and polar graphs

Polar Pattern

Fig. 2.3(a) Radiation lobes and beamwidths of an antenna pattern

Linear Pattern

Fig. 2.3(b) Linear plot of power pattern and its associated lobes and beamwidths

.Field Regions

. Reactive near field

energy stored not radiated

D³
R 0.62



λ = wavelength

D= largest dimension of the antenna

.Field Regions

. Radiating near field (Fresnel) radiating fields predominate pattern still depend on R radial component may still be appreciable

0.62

 R  2

λ = wavelength

D= largest dimension of the antenna

.Field Regions

. Far field (Fraunhofer)

field distribution independent of R

field components are essentially transverse

D²
R  2



.Radian

2 radians in full circle arc length of circle

 r 

Fig. 2.10(a) Geometrical arrangements for defining a radian

.Steradian

one steradian subtends an area of

A  r²

4π steradians in entire sphere

dA  r²sin d d

dA d   sin d d

r²

Fig. 2.10(b) Geometrical arrangements for defining a steradian.

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